1. Field of the Invention
The present invention relates to a magnetic carrier to be used in an image-forming method for visualizing an electrostatic image by employing an electrophotographic method, and to a two-component developer using the carrier.
2. Description of the Related Art
In general, the following method has heretofore been employed as an image-forming method of an electrophotographic system. An electrostatic latent image is formed on an electrostatic latent image-bearing member with various processes and then toner is caused to adhere to the electrostatic latent image to develop the electrostatic latent image. At the time of the development, the following two-component development system has been widely adopted. A carrier particle called a magnetic carrier is mixed with the toner, the toner is provided with a proper quantity of positive or negative charge by subjecting the mixture to triboelectric charging, and the development is performed with the charge as a driving force.
The two-component development system has an advantage such as good controllability of the performance of a developer because of the following reason. The system can impart functions such as the stirring, conveyance, and charging of the developer to the magnetic carrier, and hence functions are clearly shared between the carrier and the toner.
Meanwhile, in association with the evolution of a technology in the field of electrophotography, not only an increase in speed of an apparatus and the lengthening of its lifetime but also an improvement in definition of an image formed with the apparatus and the stabilization of the quality of the image have started to be required more and more strictly in recent years.
In view of the foregoing, in Japanese Patent Application Laid-Open No. 2011-33861, an attempt has been made to improve the developability of a magnetic carrier and the following magnetic carrier has been proposed. Conductive fine particles are added to the surface of the magnetic carrier to reduce the surface resistance of the magnetic carrier. Although the reduction of the resistance achieves the improvement of the developability, the reduction causes the following problem of leakage in some cases. Charge is injected into an electrostatic image from a developer carrying member through the magnetic carrier to disturb the electrostatic image. In addition, in some cases, the reduction causes the so-called solid carrier adhesion in which the magnetic carrier becomes identical in polarity to toner and hence the magnetic carrier as well as the toner adheres to the image portion of an electrostatic latent image-bearing member.
Meanwhile, Japanese Patent Application Laid-Open No. 2010-102190 proposes an increase in thickness of a coating resin layer and the formation of a two-layer coating resin that lead to the suppression of the leakage and fogging.
The magnetic carrier can suppress the leakage and the fogging. However, increasing the thickness of the coating resin layer tends to be liable to cause the coalescence of particles of the magnetic carrier. When a coalesced magnetic carrier exists, the coalesced magnetic carrier is typically removed by sieving. However, when the coalesced magnetic carrier is shredded by the impact of the sieving, a magnetic carrier having a crater-like shredded surface passes a sieve to be mixed into a product in some cases. When image output is performed with such magnetic carrier for a long time period, the external additive of the toner is selectively accumulated in the crater portion of the magnetic carrier. Accordingly, the charge-providing performance of the magnetic carrier is impaired and hence a fluctuation in color occurs in some cases. In addition, the resistance of the magnetic carrier increases owing to the increased thickness of the coating resin layer, and hence its developability reduces and an image density reduces in some cases.
As can be seen from the foregoing, it has been urgently needed to develop a magnetic carrier satisfactory in terms of the suppression of leakage and solid carrier adhesion, charge-providing performance, and developability.